Cross correlator with automatic rotational alignment

ABSTRACT

This invention relates to cross-correlation with automatic rotational alignment of images. The correlation spot is distinguished from the background illumination by rotation of the transparency out of the optimum alignment position, whereby the correlation spot disappears into the background illumination.

United 9 States Patent [191 Gamertsfelder et al.

CROSSCORRELATOR WITH AUTOMATIC ROTATIONAL ALIGNMENT Inventors: George R. Gamertsielder,

Pleasantville; Lester I. Goldflscher, New Rochelle; John K. McKendry, Pleasantville; Richard M. Vesper, Bronx, all of NY.

The Singer Company, New York, N.Y.

Filed: Apr. 22, 1963 Appl. No.: 275,475

Assignee:

US. Cl. 343/100 CL, 343/5 MM, 250/219,

235/181 Int. Cl. H041) 7/00 Field of Search 250/219, 219.1;

88/14 E; 343/5 MM, 100, 100 CL; 235/181; 340/149; 179/1 VS Primary Examiner-Carl D. Quariforth Assistant Examiner-1 A. Nelson Attorney- S. A. Giarratana and T. W. Kennedy 57 ABSTRACT This invention relates to cross-correlation with automatic rotational alignment of images. The correlation spot is distinguished from the background illumination by rotation of the transparency out of the optimum alignment position, whereby the correlation spot disappears into the background illumination.

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CROSS-CORRELATOR WITH AUTOMATIC ROTATIONAL ALIGNMENT This invention relates generally to cross-correlators.

Correlators are, in general, used to determine that relative position of two superposed objects, or images thereof, at which they are most nearly alike. Although many kinds of cross-correlators have heretofore been used, there is a need for an improved apparatus which automatically determines the best match, in both translation and rotation.

Consider a specific application of a correlator where it is desired to obtain a position fix in an aircraft. One image may be that of the actual terrain beneath the aircraft, while the other image may be an aerial photograph of the same area. If these images can be aligned or their misalignment quantitatively determined automatically and continuously in translation (x and y) and azimuth (6), not only will a position fix be obtained but the course of the aircraft can be observed and controlled as desired.

Various correlators have been proposed in the past. For example, the Berger U.S. Pat. No. 2,787,188 describes a system in which collimated light is transmitted through two spaced-apart transparencies and then condensed onto a plane. Photoelectric sensors in this plane control a servomechanism which positions the sensors so as to indicate the misalignment of the two transparencies. As another example, the co-pending application of George R. Gamertsfelder, Ser. No. 220,126, filed Aug. 27, 1962 for CORRELATOR, which is assigned to the same assignee as is the instant application, describes a system requiring neither lenses nor curved mirrors in which a sensing mechanism is automatically positioned to indicate the degree of mismatch. This system may employ electromechanical wave energy in the visible, infrared or microwave regions. Another ar rangement is described in the co-pending application of George R. Gamertsfelder, Ser. No. 215,569, filed Aug. 8, 1962 for METHOD AND APPARATUS FOR ELIMINATING CORRELATION INTERFERENCE,- which is also assigned to the same assignee as is the instant application. The latter-identified application describes a system which determines the position along the length of an extended film strip at which a shorter reference film strip achieves the best match. This disclosure recognizes that interference due to dimensional limitation, or background illumination, tends to obscure the indication of correlation and describes one solution to the problem. Briefly, the solution requires the preparation of an additional film strip having uniform light transmission characteristics equal to the average of that of the reference strip. The new strip and the reference strip are simultaneously but separately compared with the extended strip and the results subtracted to obtain a correlation signal substantially free from interference.

As far as applicants are aware, none of the prior art correlators have been concerned with automatically aligning the images in azimuth, that is, rotationally. Additionally, the interference discussed in application Ser.

No. 215,569 becomes troublesome if uncompensated,

but the solution proposed herein is not entirely satisfactory.

It is a general object of the present invention to provide an improved correlator.

Another object is to provide a correlator in which the adverse effects of background illumination are automatically minimized.

Another object is to provide a correlator in which the images are automatically aligned in azimuth.

Another object is to provide improved apparatus for automatically determining quantitatively the misalignment of two similar images.

Another object is to provide apparatus suitable for determining the alignment of an aerial photograph with the terrain beneath.

Briefly stated, the present invention is based on the principle of cyclically diffusing the correlation spot into the background illumination, thereby placing an alternating component on that illumination which is due to the correlation function. It is thus possible to generate a signal having an alternating component which component represents the correlation function alone, free from interference due to variations in background illumination. In a preferred embodiment to be described for illustrative purposes, the diffusion is accomplished by continuous oscillation of the transparency. More specifically, the reference transparency is mounted to be rotatable about the optical axis and is oscillated thereabout continuously by an electric motor. The oscillation is at a low audio frequency rate such as 20 or 30 cycles per second and extends over a range of from about five to about 25 on either side of the neutral position. A suitable radiation sensing means is mounted for movement in two dimensions in. the detecting plane. In a manner to be more fully explained, several error signals are derived, in each of which the distorting effect of background illumination has been eliminated. These error signals are used to adjust the mean angular position of the reference frequency for optimum correlation and to position the radiation sensing means in the two dimensions of the detecting plane so that the correlation spot falls on its center.

For a clearer understanding of the invention, reference may be made to the following detailed description and the accompanying drawings, in which:

FIGS. 1, 2 and 3 are diagrams useful in explaining the invention;

FIG. 4 is a bottom view of the apparatus;

FIG. 5 is a side elevation view;

FIG. 6 is a schematic showing of apparatus for positioning the radiation sensor;

FIGS. 7, 8 and 9 are graphical representations useful in explaining the invention;

FIGS. 10a, 10b, 10c an'd 10d are schematic diagrams useful in explaining the invention;

FIGS. ll, 12 and 13 are graphical representations useful in explaining the invention;

FIG. '14 is a schematic diagram of the electric connections;

FIG. 15 is a schematic cross-section view of a light commutator; and

FIG. 16 is a developed view of the surface of the light commutator.

Referring first to FIG. I, there is shown an edge view of an object 10 which may be a variable density trans,- parency illuminated from below or an opaque pattern such as a photograph illuminated from above or the terrain beneath an aircraft. In any event the amount of light transmitted or reflected by the object 10 varies over its area in some manner such as that indicated by the curve 11.

Above the object is a plate 12 having an aperture 13 formed therein. Above the plate 12 is a screen 14. When the aperture 13 is small, approaching a pinhole in size, a faithful image of the object 10 appears on the screen 14. As the aperture 13 becomes larger, the image becomes less clear, but the light distribution continues to follow the gross pattern of the object 10, as shown by the curve 15. It is assumed that the object 10, the plate 12, and the screen 14 are all substantially perpendicular to the optical axis 16.

Referring now to FIG. 2, a variable density transparency 17, which is an image of the object 10 to a reduced scale, is placed in the aperture 13. Neglecting for a moment the image forming capabilities of the aperture 13, if the transparency 17 is aligned in both rotation and translation with the corresponding features of the object 10, and if the distances from the screen 14 to the transparency l7 and to the object 10 are properly selected with due regard for the scale factor, then some of those light rays originating at each elemental area on the object 10 will pass through corresponding areas on the transparency 17 to the center of the screen 14 causing a bright spot to appear. Other rays from elemental areas on the object 10 will follow other paths through the transparency 17 to other portions of the screen but will illuminate these parts less brightly. This is seen to be true by considering, for example, those rays emanating from a bright area of the object 10. These rays emanate in all directions, but that ray which reaches the center of the screen 14 passes through a corresponding portion of the transparency 17 which is nearly transparent while other rays reaching the screen 14 encounter denser portions of the transparency 17. Accordingly, the entire screen is illuminated to some extent, with a bright spot at the center. The variation in the amplitude of illumination is indicated by the curve 18. Translation of the transparency l7 simply translates the bright spot. Rotation of the transparency 17 causes the bright spot to become less pronounced, as indicated by the curve 19. Continued rotation causes the spot to disappear into the background. The effect is similar to defocussing, and the light formerly concentrated into a bright spot is diffused over the entire area of the screen 14.

In reality, the effects discussed in connection with FIGS. 1 and 2 both appear concurrently. FIG. 3 illustrates an arrangement in which the object 10 is assumed to be the terrain beneath an aircraft and in which the plate 12, the transparency l7, and the screen 14 are assumed to be carried by the aircraft. Additionally, a photoelectric sensor occupies the central portion of the screen 14. The curves l8 and 19, representing the correlation spot, are seen to be superimposed on the curve 15 (shown dotted) which represents the variation in illumination caused by the finite size of the aperture containing the transparency 17, herein often referred to as background illumination. It is to be noted that the amplitude of the correlation spot may be no greater, and may even be less, than the amplitude of one or more peaks of the background illumination. This makes it difficult to distinguish the correlation spot from the background illumination.

The present invention distinguishes the correlation spot from the background illumination by utilizing the above-mentioned fact that rotation of the transparency 17 out of the optimum alignment position causes the correlation spot to disappear into the background illumination. The transparency 17 is oscillated continuously about the optical axis 16, causing the correlation spot to wax and wane alternately. Since the amplitude of the background illumination is substantially unaffected by rotation, such oscillation, sometimes called jitter, places an alternating component on the amplitude of the correlation spot which component can be detected independently of the background. By means to be fully described, signals are generated which are used to adjust the mean angular position of the transparency 17 for optimum correlation. In the case of translation, instead of positioning the transparency 17 so as to place the correlation spot at the center of the sensor 20, it is preferred at present to perform the equivalent operation of positioning the sensor 20 at the correlation spot and measuring its displacement from the optical axis.

Referring now to FIGS. 4 and 5, there is shown the structure for supporting and moving the transparency 17 and the sensor 20. More particularly, there is shown a frame 21 on which is rotatably mounted a hollow cylindrical structure 22 having a face plate 23. A gear sector 24 extends around a portion of the periphery of the structure 22. A motor 25 is fastened to the frame 21 and drives a pinion 26 which meshes with the gear sector 24 so as to rotate the entire cylindrical structure. A synchro transmitter 27 is also fastened to the frame 21 and is driven by the gear sector 24 through a pinion 28 so as to generate a signal indicative of the angular position of the cylindrical structure 22.

The transparency 17 is fastened to a ring 31 which is rotatable in the face plate 23. The ring 31 and the transparency 1 1 are oscillated through a small angle on the order of from 5 to 25 on either side of neutral by an electric motor 32 fastened to the cylindrical structure 22. The motor 32 drives a wheel 33 to which is fas tened a pin 34 which in turn is engaged by one end of an arm 35, the other end of which engages a pin 36 fastened to the ring 31. Rotation of the motor 32 obviously causes the ring 31 and the transparency 17 to be oscillated through a small angle with respect to the face plate 23 and cylindrical structure 22.

The radiant energy sensor 20 is mounted in a socket 42 both of which are within the cylinder 22 so as to exclude extraneous light. In the position shown in FIG. 5, the sensor 20 is approximately on the cylindrical axis but it is mounted to be movable in two orthogonal directions in a plane perpendicular to the axis. As best shown in FIG. 6, the socket 42 is mounted on a rod 43 which in turn is fastened to a small square block 44. The block 44 is slotted as shown to receive the stems of two T-shaped bars 45 and 46. The cross-bar 47 of the bar 45 is formed with rack teeth 48 which are engaged by a pinion 49 driven by a motor 51. A synchro transmitter 52 is geared by a pinion 53 to the rack teeth 48 so as to generate a signal indicative of the position of the sensor 41 along one coordinate axis. A motor 54 and a synchro transmitter 55 are similarly operatively connected through pinions 56 and 57 to rack teeth (not shown) on the cross-bar 58 of the bar 46. It is obvious that rotation of the x and y position motors 51 and 54 can position the sensor 20 in two dimensions, x and y, and that the synchro transmitters 52 and 55 generate signals indicative of such position. It is to be understood that the showing of FIG. 6 is intended to be schematic only and that any apparatus suitable for positioning the sensor 20 in x and y directions may be used.

It is preferred at present that the light sensor 20 be of the kind which delivers three unidirectional voltages, the first being indicative of the total radiation falling upon the sensitive area of the sensor and the other two having amplitudes and polarities indicative of the power and of the position, in orthogonal directions, of the centroid of an incident spot of light and both of the latter two being zero when the spot is at the center of the cell. One sensor of this kind which has been found to be suitable is designated a Radiation Tracking Transducer, type XY-ZOB RTT, made by Micro Systems, Inc., San Gabriel, California. In this unit the three voltages appear across three separate pairs of terminals, as will be more fully discussed.

It will be recalled from the discussion of FIGS. 2 and 3 that the amplitude of the correlation spot varies as the transparency I7 is rotated in azimuth. The nature of this variation is shown more clearly in FIG. 7 wherein the curve 61 depicts this variation as a function of rotation. The abscissa represents the angular displacement while the ordinate represents total light flux. As indicated in FIG. 7, the entire correlation spot may represent but a small part of the total light reaching the detecting plane. The amplitude of this spot is a maximum when the transparency I7 is in the aligned position, represented by the vertical axis 62, and decreases as it is rotated in either direction from this position. If the transparency I7 be oscillated in accordance with the curve 63 about the aligned position, the light will vary as shown by the curve 64. It is apparent that the light goes through two cycles of amplitude variation for each cycle of rotational displacement or, in other words, the principal component of amplitude variation is at twice the frequency of the mechanical oscillation. Small amplitudes of higher order harmonics are also present but the second harmonic component is the one of present interest.

If the transparency be initially displaced in one direction from the aligned position, for example to the position indicated by the line 65 of FIG. 6, and then oscillated about this position as indicated by the curve 66, the amplitude of the correlation spot on the sensor 26 will vary at the same frequency as the mechanical oscillation, as shown by the curve 67. Since the correlation function is not linear, second and higher order harmonics are also present in curve 67, but the fundamental component predominates and is of principal interest for present purposes.

If the transparency be initially displaced in the opposite direction from the aligned position, for example to the position indicated by the line 66 of FIG. 9, and then oscillated, as indicated by the curve 69, the amplitude of the correlation spot will again vary predominantly at the same frequency as the mechanical oscillation, as shown by the curve 70. However, the phase of this variation is opposite to that of the curve 67 of FIG. 8.

As the mean position of oscillation approaches the aligned position, part of the excursion will include the aligned position and the amplitude variation will include components at many frequencies including both the fundamental frequency, f, of the mechanical oscillation and the second harmonic, 2f. This is the usual condition of operation.

It is apparent, then, that as the mean position of oscillation varies from one side of the aligned position to the other, the fundamental component undergoes phase reversal as the aligned position is crossed. At the aligned position the fundamental component becomes zero, leaving variations predominantly at the second harmonic although including some higher order harmonics. Accordingly, if that signal generated by the sensor 20 which is indicative of the total light be synchronously detected against a fundamental frequency reference, there will be produced a unidirectional error signal the amplitude and polarity of which are indicative of the extent and direction of the rotational, or azimuth, misalignment.

It is to be noted that the sensor 26 generates a unidirectional signal which is due not only to the correlation spot but due also to the background illumination. However, since the background illumination does not have an alternating component, synchronous detection discriminates against the background portion of the signal and produces a signal containing correlation information only. This is true whether the correlation spot be located in a valley of the background, as illustrated in FIG. 3, or whether it be superimposed on a peak.

It is also noted that, in the region where the azimuth alignment is nearly correct, small changes in azimuth, although introducing a fundamental component, do not greatly affect the amplitude of the second harmonic component.

Consider now how the translation, or x and y, misalignment error signals are generated. Let us assume that the transparency 17 is initially aligned in azimuth but that the correlation spot is positioned off the center of the sensor 26, for example at the point P of FIG. Illa. The intensity will be greatest at the point P, diminishing in all directions as indicated by the circles surrounding the point P. Let us now assume that the transparency I7 is oscillated as shown in FIG. Ill wherein the abscissa is time, the ordinate is angular displacement, and the curve shows one cycle of the oscillation. Initially the transparency has no angular displacement, corresponding to point a, and the illumination is as shown in FIG. Mia. As the transparency 17 starts to rotate, the correlation spot becomes more diffuse, disappearing into the background as shown by FIG. lltlb by the time the transparency has turned through an angle corresponding to point b of the curve 75. When point c is reached, the spot again becomes bright as shown by FIG. lltlc. At point d, the light :is again diffused as shown by FIG. 116d. Finally the transparency is returned to point a, the illumination is as shown in FIG. a, and the cycle is complete.

The x signal generated by the sensor 26, it will be recalled, is a unidirectional voltage the polarity of which depends upon the direction of the displacement of the centroid of the incident light from the y axis. This signal has a component due to background illumination and a correlation component. The latter component varies as shown by the curve 76 of FIG. 12, wherein points a, b, c and d correspond to the positions a, b, c and d of the transparency 17. At points a and c the signal is a maximum, while at points b and d the signal is substantially zero inasmuch as the correlation spot is completely diffused, as shown by FIGS. 1l0b and Mid. It is obvious that the x signal varies in amplitude at twice the frequency of the oscillation of the transparency 17.

If the point P should move closer to the y axis, the amplitude of the signal variations would be reduced, as indicated by the curve 77 (shown dashed) of FIG. 12.

If the point P should be to the left of the y axis, the signal generated would be reversed in polarity, as

shown by the curve 78 of FIG. 13. It is to be noted that the phase of the alternating component of this signal is opposite to the phase of the alternating component of the signal 7 6 of FIG. 12.

It is apparent that the signal indicative of the x position of the correlation spot has an alternating component the frequency of which is twice that of the oscillation of the transparency and the phase of which is determined by the sign of the displacement. If this signal be synchronously detected using a reference voltage having a frequency twice the oscillation frequency, there will be obtained a unidirectional error signal the polarity and magnitude of which are indicative of the sense and extent of the displacement of the spot from the y axis.

The y-axis displacement signal is in all respects comparable to that of the x axis signal above described.

As previously mentioned, the signal developed at the x terminals of the sensor is a voltage representing, by its magnitude and polarity, the extent and direction of the deviation from the y axis of the centroid of illumination. This centroid and the signal generated thereby is made up of a unidirectional or steady component indicative of background illumination and an alternating component indicative of correlation. Examination solely of the alternating component yields a signal free of background information which is indicative of the deviation of the correlation spot from the y axis. Similar comments apply to the y signal.

Referring now to FIG. 14, the sensor 20 is shown schematically as comprising two terminal groupings, 20a and 20b. The former includes the pair of terminals 81 and 82 across which the ac voltage is developed and the pair of terminals 83 and 84 across which the y voltage is developed. The x terminals 81 and 82 are connected to an amplifier 85 which rejects direct current signals and amplifies only those frequencies at or near 2f, twice the frequency of rotation of the azimuth oscillation motor 32. The output of the amplifier 85 is connected to a synchronous detector 86, keyed by a voltage having a frequency 2f and a constant phase with respect to the azimuth oscillations. The output of the synchronous detector 86 is, of course, a unidirectional error signal the magnitude and polarity of which are indicative of the amount and sense of the displacement of the correlation light spot from the center of the sensor 20 in the x direction. This error signal controls a servo amplifier 87 which in turn controls the previously mentioned x position motor 51 so as to align the sensor 20 in the x direction with the correlation spot.

The voltage at the y terminals 83 and 84 operates through a similar chain of components including an amplifier 88, a synchronous detector 89, and a servo amplifier 91 to control the y position motor 54 so as to align the sensor 20 in the y direction with the correlation spot.

The terminal grouping 20b of the sensor 20 includes four terminals 92, 93, 94 and 95 which are connected by means of resistors 96, 97, 98 and 99, respectively, toa junction 101. In this way the voltages appearing on the terminals 92, 93, 94 and 95 are combined to form a single resulting voltage with respect to a common terminal 102. A resistor 103 interconnects the junction 101 and the common terminal 102. The voltage across the resistor 103 is indicative of the total light reaching the sensitive area of the sensor 20 and includes a unidirectional background component and alternating correlation components at both the fundamental frequencyfand the second harmonic 2fof the azimuth oscillation. The junction 101 and the terminal 102 are connected to a broadband amplifier 105 which rejects the unidirectional component and raises the level of the alternating components. The output of the amplifier 105 is connected to the input of a tuned amplifier 106 which amplifies the second harmonic component'and is also connected to the input of a tuned amplifier 107 which amplifies the fundamental component. As previously mentioned, the second harmonic component is reasonably constant in amplitude for small changes in azimuth although varying with total light level. Accordingly, this component is suitable for an automatic gain control (AGC) reference. The output of the amplifier 106 is rectified and smoothed by a detector circuit 108 and is then applied, through a bus 109, to the amplifiers 85, 88 and 107.

The output of the amplifier 107 is applied to a synchronous detector 111 which is keyed by a voltage at the fundamental frequency and having a constant phase with respect to the azimuth oscillations. The output of the synchronous detector 111 is a unidirectional error signal indicating, by its polarity and magnitude, the direction and extent of azimuth misalignment. This error signal is applied to a servo amplifier 112, the output of which controls the azimuth position motor 25 so as to correct any misalignment.

As shown in FIG. 5, the azimuth oscillation motor 32 drives a light commutator 121 which is also fastened to the cylindrical structure 22. As shown schematically in FIGS. 15 and 16, the commutator 121 comprises a drum 122 formed with slots 123, 124 and 125 in the periphery which revolves around a light source 126. A photoelectric sensor 127 is positioned adjacent to the slot 123 while a similar sensor 128 is positioned adjacent to the slots 124 and 125. The sensors 127 and 128 therefore generate alternating voltages at the fundamental and second harmonic frequencies, respectively.

Referring again to FIG. 14, the fundamental frequency voltage is applied to an amplifier 131 the output of which controls, or keys, the synchronous detector 111. Similarly, the voltage at the second harmonic frequency, 2f, is amplified by an amplifier 132 the output of which keys the synchronous detectors 86 and 89.

It is apparent from the foregoing description that the correlator of the present invention provides a number ofadvantages. Not only is the radiant energy sensor automatically aligned in x and y, but the reference transparency is automatically aligned in azimuth. The error signals for controlling the alignment are free from the distortion caused by background illumination. The synchro transmitters 27, 52 and 55 generate signals which can be used for navigation or other purposes.

It is to be noted that the principles of the present invention are applicable to correlators employing radiant energy in the ultraviolet, visible, infrared and radio frequency portions of the spectrum. The reference transparency must, of course, be selected to, suit the radiant energy being used.

As previously mentioned, the effect of the rotation of the transparency 17 is to diffuse the correlation spot so that the sensor 20 periodically receives illumination corresponding to background illumination only. It would be possible, of course, to devise other schemes for periodically diffusing the radiant energy but at present the azimuth oscillation arrangement is preferred because this arrangement not only diffuses the correlation spot, as required for distinguishing the correlation function from the background, but at the same time provides azimuth sensing, thereby eliminating the need for additional azimuth sensing equipment.

Although a preferred embodiment has been described for illustrative purposes, many modifications can be made within the spirit of the invention. It is therefore desired that the protection afforded by Letters Patent be limited only by the true scope of the appended claims.

What is claimed is:

1. In a correlator in which radiant energy is transmitted from an object through a variable density planar representation thereof to a detecting plane,

apparatus for determining the rotational misalignment of said object and said representation, comprising,

a radiant energy sensor positioned in said detecting plane for generating a unidirectional voltage indicative of the total amount of radiant energy falling thereon,

means for subjecting said representation to oscillatory rotary motion about an axis perpendicular thereto,

whereby said voltage varies in amplitude and includes a component varying at the frequency of said oscillatory rotary motion,

means for generating an alternating voltage at the frequency of and in synchronism with said oscillatory rotary motion, and

synchronous detector means controlled by said alternating voltage for generating a unidirectional error signal voltage the magnitude and polarity of which are indicative of the magnitude and phase respectively of said component with respect to said alternating voltage.

2. In a correlator in which radiant energy is transmitted from an object through a variable density planar representation thereof to a detecting plane,

apparatus for determining the translational misalignment of said object and said representation, comprising,

radiant energy sensing means positioned in said detecting plane for generating first and second unidirectional voltages, the magnitude and polarity of each of which is indicative of the amount and sense of the displacementof a spot of light from the center of said means in one of two orthogonal directions,

means .for subjecting said representation to oscillatory rotary motion about an axis perpendicular thereto,

whereby each of said voltages varies in amplitude and includes a component varying at twice the frequency of said oscillator rotary motion,

means for generating an alternating voltage at twice the frequency of and in synchronism with said oscillatory rotary motion, and

two synchronous detector means each controlled by said alternating voltage for generating first and second error signal voltages the magnitude and polarity of which are indicative of the magnitude and phase of said components of said first and second unidirectional voltages respectively with respect to said alternating voltage.

3. In a correlator for indicating the amount of mismatch between an illuminated body and a partially transparent image thereof located in spaced parallel relation thereto in which such mismatch is determined by a radiant energy sensing means positioned to receive radiation transmitted from said body through said image, the improvement which comprises,

means for oscillating said image about an axis perpendicular thereto,

whereby said sensing means generates signals having alternating current components at both the fundamental and the second harmonic of the frequency of oscillation,

means for generating first and second voltages in synchronism with the oscillation of said image having frequencies equal to and twice that of said oscillation respectively, and

means for synchronously detecting said signals using each of said first and second voltages as a reference,

whereby error signals indicative of mismatch are generated.

4. A correlator for indicating the amount of mismatch between an illuminated body and a partially transparent image thereof located in spaced parallel relation thereto, comprising,

a radiant energy sensing means for generating first, second and third unidirectional voltages, said first voltage having a magnitude indicative of the total amount of radiant energy reaching said sensing means, said second and third voltages indicating by their amplitude and polarity the magnitude and sense of the displacement in orthogonal directions of a spot of incident energy from the center of said sensing means,

said sensing means being'positioned to receive radiant energy transmitted from said body through said image,

means for oscillating said image at a convenient frequency about an axis perpendicular thereto,

whereby the quality of the match between said body and said image varies periodically, and

whereby each of said voltages varies in amplitude,

means for generating fourth and fifth voltages in synchronism with the oscillation of said image and having frequencies equal to and twice that of said convenient frequency respectively,

means for synchronously detecting the variations in amplitude of said first voltage using said fourth voltage as a reference, whereby there is derived a first error signal indicative of the rotational mismatch between said body and said image, and means for synchronously detecting the amplitude variations in each of said second and third voltages using said fifth voltage as a reference, whereby there are generated second and third error signals indicative of the translational mismatch of said image and said body in said orthogonal directions; 5. In a correlator in which an illuminated body and a partially transparent image thereof are located in spaced-apart parallel relation to each other, apparatus for rotationally positioning said image to obtain the best correlation between said image and said body, comprising,

radiant energy sensing means for generating a unidirectional voltage indicative of the amount of radiant energy falling thereon,

said sensing means being located to receive radiant energy transmitted from said body through said image,

means for oscillating said image about an axis perpendicular thereto, whereby said unidirectional voltage varies in amplitude,

means for generating an alternating voltage in synchronism with and at the frequency of the oscillation of said image, means for synchronously detecting the variations in amplitude of said unidirectional voltage by employing said alternating voltage as a reference, whereby there is obtained a unidirectional error signal the polarity and amplitude of which are indicative of the sense and magnitude of the rotational misalignment of the mean position of said image, and

means responsive to said error signal for rotationally positioning said image so as to minimize said error signal.

6. A correlator for indicating the translational mismatch between an illuminated body and a partially transparent image thereof located in spaced parallel relation thereto, comprising,

radiant energy sensing means for generating first and second unidirectional voltages the amplitude and polarity of each of which are indicative of the amount and sense of the displacement of a spot of radiant energy from the center thereof in one of two orthogonal directions,

said sensing means being located to receive radiant energy transmitted through said body through said image,

means for oscillating said image about an axis perpendicular thereto, whereby said unidirectional voltages vary in amplitude,

means for generating an alternating voltage in synchronism with and at twice the frequency of the oscillation of said image,

means for synchronously detecting the variations in amplitude of each of said unidirectional voltages by employing said alternating voltage as a reference, whereby there are obtained two unidirectional error signals the amplitude and polarity of each of which are indicative of the magnitude and phase of the amplitude variations of one of said unidirectional voltages,

means controlled by said error signals for moving said sensing means in said 0 

1. In a correlator in which radiant energy is transmitted from an object through a variable density planar representation thereof to a detecting plane, apparatus for determining the rotational misalignment of said object and said representation, comprising, a radiant energy sensor positioned in said detecting plane for generating a unidirectional voltage indicative of the total amount of radiant energy falling thereon, means for subjecting said representation to oscillatory rotary motion about an axis perpendicular thereto, whereby said voltage varies in amplitude and includes a component varying at the frequency of said oscillatory rotary motion, means for generating an alternating voltage at the frequency of and in synchronism with said oscillatory rotary motion, and synchronous detector means controlled by said alternating voltage for generating a unidirectional error signal voltage the magnitude and polarity of which are indicative of the magnitude and phase respectively of said component with respect to said alternating voltage.
 2. In a correlator in which radiant energy is transmitted from an object through a variable density planar representation thereof to a detecting plane, apparatus for determining the translational misalignment of said object and said representation, comprising, radiant energy sensing means positioned in said detecting plane for generating first and second unidirectional voltages, the magnitude and polarity of each of which is indicative of the amount and sense of the displacement of a spot of light from the center of said means in one of two orthogonal directions, means for subjecting said representation to oscillatory rotary motion about an axis perpendicular thereto, whereby each of said voltages varies in amplitude and includes a component varying at twice the frequency of said oscillator rotary motion, means for generating an alternating voltage at twice the frequency of and in syncHronism with said oscillatory rotary motion, and two synchronous detector means each controlled by said alternating voltage for generating first and second error signal voltages the magnitude and polarity of which are indicative of the magnitude and phase of said components of said first and second unidirectional voltages respectively with respect to said alternating voltage.
 3. In a correlator for indicating the amount of mismatch between an illuminated body and a partially transparent image thereof located in spaced parallel relation thereto in which such mismatch is determined by a radiant energy sensing means positioned to receive radiation transmitted from said body through said image, the improvement which comprises, means for oscillating said image about an axis perpendicular thereto, whereby said sensing means generates signals having alternating current components at both the fundamental and the second harmonic of the frequency of oscillation, means for generating first and second voltages in synchronism with the oscillation of said image having frequencies equal to and twice that of said oscillation respectively, and means for synchronously detecting said signals using each of said first and second voltages as a reference, whereby error signals indicative of mismatch are generated.
 4. A correlator for indicating the amount of mismatch between an illuminated body and a partially transparent image thereof located in spaced parallel relation thereto, comprising, a radiant energy sensing means for generating first, second and third unidirectional voltages, said first voltage having a magnitude indicative of the total amount of radiant energy reaching said sensing means, said second and third voltages indicating by their amplitude and polarity the magnitude and sense of the displacement in orthogonal directions of a spot of incident energy from the center of said sensing means, said sensing means being positioned to receive radiant energy transmitted from said body through said image, means for oscillating said image at a convenient frequency about an axis perpendicular thereto, whereby the quality of the match between said body and said image varies periodically, and whereby each of said voltages varies in amplitude, means for generating fourth and fifth voltages in synchronism with the oscillation of said image and having frequencies equal to and twice that of said convenient frequency respectively, means for synchronously detecting the variations in amplitude of said first voltage using said fourth voltage as a reference, whereby there is derived a first error signal indicative of the rotational mismatch between said body and said image, and means for synchronously detecting the amplitude variations in each of said second and third voltages using said fifth voltage as a reference, whereby there are generated second and third error signals indicative of the translational mismatch of said image and said body in said orthogonal directions.
 5. In a correlator in which an illuminated body and a partially transparent image thereof are located in spaced-apart parallel relation to each other, apparatus for rotationally positioning said image to obtain the best correlation between said image and said body, comprising, radiant energy sensing means for generating a unidirectional voltage indicative of the amount of radiant energy falling thereon, said sensing means being located to receive radiant energy transmitted from said body through said image, means for oscillating said image about an axis perpendicular thereto, whereby said unidirectional voltage varies in amplitude, means for generating an alternating voltage in synchronism with and at the frequency of the oscillation of said image, means for synchronously detecting the variations in amplitude of said unidirectional voltage by employing said alternating voltage as a reference, whereby there is obtained a unIdirectional error signal the polarity and amplitude of which are indicative of the sense and magnitude of the rotational misalignment of the mean position of said image, and means responsive to said error signal for rotationally positioning said image so as to minimize said error signal.
 6. A correlator for indicating the translational mismatch between an illuminated body and a partially transparent image thereof located in spaced parallel relation thereto, comprising, radiant energy sensing means for generating first and second unidirectional voltages the amplitude and polarity of each of which are indicative of the amount and sense of the displacement of a spot of radiant energy from the center thereof in one of two orthogonal directions, said sensing means being located to receive radiant energy transmitted through said body through said image, means for oscillating said image about an axis perpendicular thereto, whereby said unidirectional voltages vary in amplitude, means for generating an alternating voltage in synchronism with and at twice the frequency of the oscillation of said image, means for synchronously detecting the variations in amplitude of each of said unidirectional voltages by employing said alternating voltage as a reference, whereby there are obtained two unidirectional error signals the amplitude and polarity of each of which are indicative of the magnitude and phase of the amplitude variations of one of said unidirectional voltages, means controlled by said error signals for moving said sensing means in said orthogonal directions so as to reduce said error signals to zero, and means for generating signals indicative of the position of said sensing means with respect to a reference position. 